Rectangular plate under loading pins

[melixman2016]

Hi,
While 4-bending tests on a sandwich we got Identation to avoid that we use a small rectangular plate under loading pins.
can this affect the outcome of the Test?
Thank you

 

[Compositepro]

You should used larger diameter pins rather than flats. Flats will have sharp edges that will act as stress risers. That tests for bearing strength rather than pure shear in the core. However, a test with flats could be more realistic for your application, and is better than using pins that are too small.

 

[melixman2016]

Thank you for your helpful information
and at which size is pins small or large?

 

[Compositepro]

Ideally the pin diameter is slightly smaller than the curvature of the test specimen at max load. Too small a pin causes crushing or dimpling (deformation) of the skin, as you have observed. I believe 1/4" or 1/2" diameter is fairly standard. This kind of information is usually in the ASTM test methods.

 

[SWComposites]

There should be a section in the ASTM standard that defines this.

 

[melixman2016]

Thank you very much
it is a formula for minimum area to avoid indentation
"An Introduction to Sandwich Construction" by Dan Zenkert Page 111